Optical members and compositions for producing them

ABSTRACT

It is disclosed a polymerizable composition for producing an optical member for 850 nm wavelength comprising: a polymerizable monomer composition and a compound, having a different refractive index from that of the polymerizable monomer composition, whose structure has a benzene ring substituted by a substituent having a Hammett value of not greater than 0.04 or by plural substituents having an average value of Hammett values thereof of not greater than 0.04. It is also disclosed a polymerizable composition for producing an optical member comprising a polymerizable monomer composition comprising at least one selected from the group consisting of C 7-20  alicyclic (meth) acrylates and a compound, having a different refractive index from that of the polymerizable monomer composition and having a solubility parameter of not greater than 10.9, whose structure has a benzene ring substituted by the substituent or the substituents defined above.

TECHNICAL FIELD

The present invention belongs to a technical field of plastic opticalmembers, in particular belongs to a technical field of plastic opticalmembers preferably applicable to plastic optical fibers, light guides oroptical lenses, and polymerizable compositions for producing the plasticoptical members.

RELATED ART

In recent years, plastic optical members are widely used for variousapplications including optical fibers, light guides and optical lenses,by virtue of its advantages such that allowing more simple producing andprocessing at a lower cost as compared with quartz-base optical membershaving the same structure. The plastic optical fiber is slightlyinferior to quartz-base fiber since the entire region of the elementfiber thereof is made of plastic material and has, as a consequence, alittle larger transmission loss, but superior to the quartz-base opticalfiber in that having an excellent flexibility, lightweight property,workability, better applicability in producing a large core diameterfiber and a lower cost. The plastic optical fiber is thus studied as atransmission medium for optical communication which is effected over adistance relatively as short as allowing such large transmission loss tobe ignored (ref. pages from 1 to 8 of “Plastic Optical Fiber” publishedby KYORITSU SHUPPAN CO., LTD. in 1997, and edited by POF Consortium).

The plastic optical fiber generally has a center core (referred to as“core region” in the specification) made of an organic compound andcomprises a polymer matrix, and an outer shell (referred to as “cladregion” in the specification) made of an organic compound having arefractive index differing from (generally lower than) that of the coreregion. In particular, the plastic optical fiber having a gradedrefractive index along the direction from the center to the outsidethereof, namely a GI type plastic optical fiber, recently attracts agood deal of attention as an optical fiber which can ensure a hightransmission capacity. As one method for preparing such plastic opticalfibers, it has been proposed a process comprising forming a fiber basemember (referred to as “preform” in the specification) according to aninterfacial gel polymerization and then drawing the preform (Ref. pagesfrom 66 to 72 of “Plastic Optical Fiber” published by KYORITSU SHUPPANCO., LTD. in 1997, and edited by POF Consortium; WO93/08388 and thelike).

Optical transmitters are required to have little transmission loss andto have a high transmitting capacity. Especially, when plastic opticalfibers are used with a light source emitting near-IR light such as 850nm, an absorption attributed to overtone of stretching vibration ofinteratomic bonds is a factor responsible for increasing transmissionloss. It has been known that an absorption attributed to overtone of C—Hbond stretching vibration, which constitutes a matrix material of anplastic optical fiber, contributes to worsening transmission loss, andit has been often carried out replacing H atoms with heavier atoms suchas deuterium or fluorine atoms (Ref. pages from 41 to 66 of “PlasticOptical Fiber” published by KYORITSU SHUPPAN CO., LTD. in 1997, andedited by POF Consortium). Regarding known compounds which are added tothe matrix materials in order to adjust a distribution of refractiveindex and ensure a sufficient difference in refractive index between acore and a clad regions, referred to as “enhancer of refractive index”or “dopant”, which may be non-polymerizable or polymerizable compound,almost all are compounds having at least one benzene ring, and it hasbeen known that an absorption attributed to a forth overtone of C—Hstretching vibration in the benzene ring contributes to increasingtransmission loss. Thus, it has been carried out deuteration of theenhancers of refractive index as well as the matrix materials (Ref.pages from 20 to 22 of WO93/08488). The deuteration thereof can achievea remarkable reduction of transmission loss, however, there are otherproblems such that deuterated materials are generally so expensive andavailable compounds are limited to a few species.

It has been also known that compatibility between a matrix material anda dopant has significant effect on transmission loss. It has been alsoknown that hydrophobic property of the matrix material containing thedopant has effect on transmission loss since there is an absorptionattributed to overtone of OH stretching vibration within a wavelengthrange near 850 nm.

SUMARRY OF THE INVENTION

One object of the present invention is to provide a polymerizablecomposition capable of producing optical members for 850 nm wavelengthin low cost.

Other object of the present invention is to provide with low cost anoptical member having low transmission loss at 850 nm.

Other object of the present invention is to provide a polymerizablecomposition capable of producing optical members having a lowtransmission loss at 850 nm wavelength and a goodmoisture-heat-resistant property.

Other object of the present invention is to provide with low cost anoptical member having a low transmission loss at 850 nm and a goodmoisture-heat-resistant property.

The present inventors conducted various studies, and as a result, theyfound that there is a correlation between an absorption peak attributedto a fourth overtone of C—H stretching vibration in a benzene ring and aHammett value of a substituted group to the benzene ring, and that theabsorption peak is shifted to longer wavelengths when the benzene ringwas substituted by an electron-donating substituent. After furthervarious studies, such as studies of hydrophobic combinations of dopantsand matrix materials in the viewpoint of solubility parameters, on thebasis of these findings, the present invention was achieved.

In one aspect, the present invention provides a polymerizablecomposition for producing an optical member for 850 nm wavelengthcomprising:

a polymerizable monomer composition,

a polymerization initiator, and

a compound, having a different refractive index from that of thepolymerizable monomer composition, whose structure has a benzene ringsubstituted by a substituent having a Hammett value of not greater than0.04 or by plural substituents having an average value of Hammett valuesthereof of not greater than 0.04.

As embodiments of the present invention, there are provided thepolymerizable composition wherein the polymerizable monomer compositioncomprises at least one selected from the group consisting of esters of apropenoic acid and esters of derivatives thereof in a major proportion;the polymerizable composition wherein the polymerizable monomercomposition comprises at least one selected from the group consisting ofesters of a (meth) acrylic acid and esters of derivatives thereof in amajor proportion; the polymerizable monomer composition wherein thepolymerizable monomer composition comprises at least one selected fromthe group consisting of compounds including a C—F bond; and thepolymerizable composition wherein the polymerizable monomer compositioncomprises at least one selected from the group consisting of compoundsincluding a C-D (deuterium) bond.

In another aspect, the present invention provides an optical memberproduced by polymerization of the composition, so as to form a regionhaving a graded refractive index.

In another aspect, the present invention provides an optical member for850 nm wavelength comprises:

a polymer composition comprising at least one polymer selected from thegroup consisting of (meth)acrylates base polymers and

a compound having a different refractive index from that of apolymerizable monomer composition of the polymer composition wherein thecompound has an absorption peak attributed to a fourth overtone of C-Hbond stretching vibration in a benzene ring at not shorter than 875 nm.

As embodiment of the present invention, there are provided the opticalmember wherein the compound is selected from the group consisting of:

Formula (1)

wherein R¹ to R¹⁰ respectively represent a hydrogen, an alkyl, analkenyl, an alkyloxy, an alkenyloxy, or dialkylamino provided that atleast four of them represent an alkyl, alkenyl, alkyloxy, alkenyloxy ordialkylamino; the optical member which comprises a region having agraded refractive index; and the optical member which comprises a regionhaving a graded refractive index along the direction from the center tothe outside.

In another aspect, the present invention provides a polymerizablecomposition for producing an optical member comprising:

a polymerizable monomer composition comprising at least one selectedfrom the group consisting of:

Formula (2)

wherein X¹ is hydrogen (H) or deuterium (D) wherein two X¹ may be sameor different; Y¹ represents H, D, CH₃ or CD₃; and R¹ represents a C₇₋₂₀alicyclic hydrocarbon group;

a polymerization initiator, and

a compound, having a different refractive index from that of thepolymerizable monomer composition and having a solubility parameter ofnot greater than 10.9, whose structure has a benzene ring substituted bya substituent having a Hammett value of not greater than 0.04 or bysubstituents having an average value of Hammett values thereof of notgreater than 0.04.

AS embodiments of the present invention, there are provided thepolymerizable composition wherein the polymerizable monomer compositioncomprises an alicyclic hydrocarbon methyl methacrylate and methylmethacrylate in a major proportion; and the polymerizable compositionwherein the polymerizable monomer composition comprises at least onecompound including a C-D bond.

In another aspect of the present invention, an optical member producedby polymerization of the composition, so as to form a region having agraded refractive index.

As embodiments of the present invention, there are provided the opticalmember comprising a core region having a graded refractive index, whichis produced by polymerization of the composition and a clad regioncladding the core region; the optical member wherein the core regionhaving a graded refractive index along the direction from the center tothe outside; and the optical member the clad region is essentiallyformed of a polymerizable monomer composition comprising a sameingredient or same ingredients in a major portion as those of the coreregion; and the optical member which is an optical fiber, a light guideor an optical lens.

In another aspect, the present invention provides a process forproducing an optical member comprising a step of polymerizing the abovepolymerizable composition.

As embodiments of the present invention, there is provided the processwherein the step of polymerizing, the polymerization temperature is 50degrees Celsius or above.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments of the present invention are described in detail bellow.

It is to be noted that examples of the optical members produced bypolymerization of the compositions according to the present inventioninclude light guide devices such as optical fibers or light guides;lenses used for still cameras, camcorders, telescopes, glasses, plasticcontact lenses or solar collectors; mirrors such as concave mirrors orpolygon mirrors, and prisms such as pentaprism. Among these, the opticalmembers are desirable applied to light guide devices, lenses or mirrors,and more desirable applied to optical fibers, light guides or lenses.

1. Polymerizable Composition

At first, embodiments of the polymerizable composition according to thepresent invention are described in detail bellow.

1-1 First Embodiment of the Polymerizable Composition

The first embodiment of the polymerizable composition according to thepresent invention essentially comprises a polymerizable monomercomposition, a polymerization initiator capable of initiatingpolymerization thereof and a compound having a different refractiveindex from that of the monomer composition, occasionally referred to as“dopant” or “enhancer of refractive index” hereinafter. According to thefirst embodiment, the decrease of transmission loss due to a dopant canbe achieved by using a compound, having at least one benzene ringsubstituted by a substituent having a Hammett value within a particularrange or by groups having an average value of Hammett values thereofwithin the particular range, as a dopant. The composition of the firstembodiment may be used for producing optical members, in particularoptical members having a distribution in refractive index values.

Various materials used for the first embodiments are described bellow.

1-1-1 Polymerizable Monomer Composition

According to the first embodiment, the polymerizable monomer compositiondesirably comprises at least one selected from the group consisting ofesters of propenoic acid and derivatives thereof in major proportion.Embodiments of esters of propenoic acids and derivatives thereof includeacrylates and methacrylates, both of them are referred to as “(meth)acrylates” hereinafter. The term of “comprise a monomer in majorproportion” is used for not only the embodiment consisting of themonomer, but also embodiments further comprising at least onepolymerizable monomer other than the monomer so far as not loweringoptical properties. The polymerizable monomer composition may contain atleast one selected from the group consisting of (meth) acrylates and atleast one selected from the group polymerizable monomers other than(meth)acrylates such as styrene or maleimide, so as to form anycopolymers. When deuterated (meth)acrylates, in which at least a part ofhydrogens are replaced with deuteriums, are used, optical members havinglow transmission loss can be produced, and thus deuterated(meth)acrylates are desirable. Using fluorinated (meth)acrylates mayeasily result in much difference of refractive index between theobtained optical fibers and copolymers of non-fluorinated monomers, andin consequence, may easily create graded refractive index structures.Thus, fluorinated (meth)acrylates are desirable.

Here lists examples of usable (meth) acrylates in the first embodiment,however, the examples are not limited to these.

-   (a) non-fluorinated (meth)acrylates such as methyl methacrylate,    ethyl methacrylate, isopropyl methacrylate, t-butyl methacrylate,    benzyl methacrylate, phenyl methacrylate, cyclohexyl methacrylate,    diphenylmethyl methacrylate, tricyclo [5.2.1.0^(2,6)]decanyl    methacrylate, adamantyl methacrylate, isobornyl methacrylate, methyl    acrylate, ethyl acrylate, t-butyl acrylate or phenyl acrylate;-   (b) fluorinated (meth)acrylates such as 2,2,2-trifluoroethyl    methacrylate, 2,2,3,3-tetrafluoropropyl methacrylate,    2,2,3,3,3,-pentafluoropropyl methacrylate,    1-trifluoromethyl-2,2,2-trifluoroethyl methacrylate,    2,2,3,3,4,4,5,5-octafluoropentyl methacrylate or    2,2,3,3,4,4-hexafluorobutyl methacrylate are exemplified.

Polymerizable monomers other than (meth)acrylates may be used. Herelists examples of usable polymerizable monomers other than (meth)acrylates in the first embodiment, however, the examples are not limitedto these.

-   (c) styrene base compounds such as styrene, alpha-styrene,    chlorostyrene or bromostyrene;-   (d) vinyl esters such as vinyl acetate, vinyl benzoate, vinyl    phenylacetate or vinyl chloroacetate;-   (e) maleimides such as N-n-butylmaleimide, N-t-butylmaleimide,    N-isopropylmaleimide or N-cyclohexyl maleimide are exemplified.

According to the first embodiment, one compound, or two or morecompounds, selected from the group consisting of (meth)acrylates may beused as a major component of the polymerizable monomer composition. Thecontent of the compound, or the content of the two or more compounds,selected from the group consisting of (meth)acrylates is desirably notsmaller than 50 wt %, more desirably not smaller than 60 wt % mass, andmuch more desirably 70 wt %, of the total polymerizable composition, andmost desirably, all monomers contained in the polymerizable monomercomposition are selected from the group consisting of (meth)acrylates.

According to the first embodiment, as using a particular class of acompound described bellow are used for a dopant, the compositioncontaining the dopant has a greater refractive index than that of acomposition not containing the dopant, or the copolymer containing thedopant as a copolymerization monomer has a greater refractive index thanthat of a polymer not containing the dopant as a copolymerizationmonomer. Fluorinated polymerizable monomers, in which at least a part ofhydrogen atoms in C—H bonds are replaced with fluorine atoms, having atleast a C—F bond, are desirably used. In particular, any compoundsselected from the group consisting of the above-mentioned fluorinated(meth)acrylates, or any mixtures of at least one selected from the groupconsisting of the above-mentioned fluorinated (meth)acrylates andfluorinated acrylates and at least one selected from the groupconsisting of non-fluorinated (meth)acrylates are desirably used.

In order to further lower transmission loss, deuterated compounds of themonomers exemplified above may be used desirably.

1-1-2 Polymerization Initiator

The composition comprises a polymerization initiator which can initiatepolymerization of the polymerizable monomer composition. Thepolymerization initiator may be selected from known polymerizableinitiators depending on various factors such as polymerizable monomerscontained in the composition or polymerization process. The examples ofthe polymerization initiator include peroxides such as benzoyl peroxide(BPO), t-butylperoxy-2-ethylhexanate (PBO), di-t-butylperoxide (PBD),t-butylperoxyisopropylcarbonate (PBI) orn-butyl-4,4-bis(t-butylperoxy)valerate (PHV); and azo compounds such as2,2′-azobisisobuthylonitrile, 2,2′-azobis(2-methylbuthylonitrile),1,1′-azobis(cyclohexane-1-carbonitrile), 2,2′-azobis(2-methylpropane),2,2′-azobis(2-methylbutane), 2,2′-azobis(2-methylpentane),2,2′-azobis(2,3-dimethylbutane), 2,2′-azobis(2-methylhexane),2,2′-azobis(2,4-dimethylpentane), 2,2′-azobis(2,3,3-trimethylbutane),2,2′-azobis(2,4,4-trimethylpentane), 3,3′-azobis(3-methylpentane),3,3′-azobis(3-methylhexane), 3,3′-azobis(3,4-dimethylpentane),3,3′-azobis(3-ethylpentane), dimethyl-2,2′-azobis(2-methylpropionate),diethyl-2,2′-azobis(methylpropionate) ordi-t-butyl-2,2′-azobis(2-methylpropionate). Two or more polymerizationinitiators may be used in combination.

1-1-3 Chain Transfer Agent

The composition according to the first embodiment desirably contains achain transfer agent. The chain transfer agent may mainly be used foradjusting molecular weight of the obtained polymer. The chain transferagent can be properly selected in consideration of the monomer to beemployed. The chain transfer constants of the chain transfer agents forvarious monomers can be referred to publications such as “PolymerHandbook 3^(rd) edition” edited by J. BRANDRUP and E. H. IMMERGUT,published by JOHN WILEY&SON. The chain transfer constants can be alsoobtained by experimental tests according to methods disclosed in“Kohbunshi gousei no jikkenhou (Experimental methods for polymersynthesis)” written by Takayuki Ohtsu and Masaetsu Kinoshita, publishedby Kagaku-Dojin Publishing Company, INC (1972).

When methyl methacrylate is used as a polymerizable monomer, at leastone selected from the group consisting of alkylmercaptans(n-butylmercaptan, n-pentylmercaptan, n-octylmercaptan,n-laurylmercaptan, t-dodecylmercaptan, etc.) and thiophenols(thiophenol, m-bromothiophenol, p-bromothiophenol, m-toluenethiol,p-toluenethiol, etc.) is desirably used as a chian transfer agent. Amongthese, alkyl mercaptans such as n-octylmercaptan, n-laurylmercaptan ort-dodecylmercapton are preferred. It is also possible to use the chaintransfer agents in which at least a part of hydrogen atoms of C—H bondsare replaced with deuterium atoms. Two or more chain transfer agents maybe used in combination.

1-1-4 Dopant: Enhancer of Refractive Index

The polymerizable composition according to the first embodiment containsa compound having a different refractive index from that of thepolymerizable monomer composition. The dopant may be polymerizable ornon-polymerizable. When a polymerizable dopant is used, it may be moredifficult to adjust various properties since the copolymerization of thepolymerizable monomer composition and the dopant may be carried out,however, the advantage in heat resistant property may be obtained. Thedopant is also referred to as enhancer of refractive index, and acompound having a property that increases the refractive index of acomposition containing it as compared with a composition not containingit, or increases the refractive index of a copolymer containing it as acopolymerization component as compared with a polymer not containing it.The difference in refractive index between the composition containingthe dopant and the composition not containing the dopant is desirablynot smaller than 0.001.

According to the first embodiment, at least one compound selected fromthe group consisting of benzene derivatives having a benzene ringsubstituted by at least one substituent is used as a dopant. The presentinventors found that there is a negative correlation between the Hammettvalue of the substituent and the wavelength of the absorption peakattributed to the fourth overtone of C—H stretching vibration in thebenzene ring. In other words, they found that the absorption peakattributed to the fourth overtone of C—H bond stretching vibration in abenzene ring is influenced by the substituents of other carbon atoms inthe benzene ring, and that the absorption peak attributed to the fourthovertone of the stretching vibration is shifted to longer wavelengths asthe Hammett values thereof are smaller. In order to lower transmissionloss at 850 nm, it is preferred that the absorption peak attributed tothe fourth overtone of C—H stretching vibration is shifted to longerwavelengths so that the absorption peak or the foot of the absorption isfar from 850 nm.

For preventing the dopant from influencing absorption at 850nm, thedopant desirably has the absorption peak at not shorter than 875 nm,more desirably at not shorter than 877 nm and much more desirably at notshorter than 880 nm. When the dopant having an absorption peak at notshorter than 880 nm is used, transmission loss may hardly occur due tothe absorption of a transmitting light of 850 nm.

In order to shift the absorption peak attributed to the fourth overtoneof C—H stretching vibration to longer wavelengths, the Hammett value ofa substituent or plural substituents is desirably not greater than 0.04,more desirably not greater than −0.05 and much more desirably notgreater than −0.1. The minimum of the Hammett value is desirably −0.6.In addition, by introduction of such a substituent or such substituents,the number of C—H bonds is decreased, and transmission loss due to anabsorption attributed fourth overtone of C—H stretching vibration itselfmay be further lowered as a secondary effect.

In the specification, when there is a substituent, the Hammett valuemeans the Hammett constant of the substituent as described in ChemicalReviews, Vol.91, No.2, pp.168-175(1991). The same Hammett constant isgenerally used for both of substituting at ortho position and at paraposition, on the other hand, the different Hammett constant is used forsubstituting at meta position. For example, given a benzene compoundsubstituted by a substituent, there are two hydrogen atoms respectivelyat ortho position and is one hydrogen atom at para position with respectto the position of the substituent R¹. When the Hammett constant of R¹at both of para and ortho positions is σ_(1p) and the Hammett constantof R¹ at meta position is σ_(1m), the Hammett value σ can be calculatedwith the following formula:σ=(σ_(1p)×3+σ1m×2)/5

On the other hand, when there are plural substituents, the Hammett valuemeans an average value of Hammett values thereof. The process forcalculating the Hammett value when there are plural substituents will bedescribed hereinafter with examples respectively having one benzene ringand two benzene rings. It is noted that the Hammett constant of R^(i),when i is any positive number, substituting at meta position is referredto as σ_(1m) and the Hammett constant of R^(i) substituting at eitherpara or ortho position is referred to as σ_(1p), in other words, theHammett constant for para position is used for ortho position.

Calculating method when there is one benzene ring:

In the above structure, the sum of the Hammett values of substituentsR¹, R² and R³, for each positions a, b and c is:σ_(a)=σ_(1p)+σ_(2m)+σ_(3m);σ_(b)=σ_(1m)+σ_(2p)+σ_(3p);σ_(c)=σ_(1m)+σ_(2p)+σ_(3p).

The average value of σ₁, σ_(b) and σ_(c), i.e. the sum average thereof,which is calculated according to the following formula, is the Hammettvalue of the plural substituents which the above compound has.σ=(σ_(a)+σ_(b)+σ_(c))/3

Calculating method when there are two benzene rings substituents:

In the above structure, the sum of the Hammett values of substituentsR¹, R², R³, R⁴ and —S—Ar group for each the positions a, b, c, d, e andf is:σ_(a)=σ_(1p)+σ_(2m)+σ_((Sph)m);σ_(b)=σ_(1m)+σ_(2p)+σ_((Sph)p);σ_(c)=σ_(1m)+σ_(2p)+σ_((Sph)p);σ_(d)=σ_(3m)+σ_(4p)+σ_((Sph)p);σ_(e)=σ_(3p)+σ_(4m)+σ_((Sph)m);σ_(f)=σ_(1m)+σ_(2p)+σ_((Sph)p).

It is noted that the Hammett constant of —S—Ph, i.e. σ_((Sph)m) orσ_((Sph)p), where Ph represents a non-substituted benzene ring, is usedfor —S—Ar, where Ar represents substituted phenyl, with regarding all—S—Ar as —S—Ph.

The average value of σ_(a) to σ_(f), i.e. the sum average thereof, whichis calculated according to the following formula, is the Hammett valueof the plural substituents that the above compound has.σ=(σ₁+σ_(b)+σ_(c)+σ_(d)+σ_(e)+σ_(f))/6

Here lists examples of benzene derivatives substituted by onesubstituent or plural substituents having a Hammett value of not greaterthat 0.04.

Among the compounds having a benzene ring substituted with onesubstituent or plural substituents having a Hammett value of not greaterthan 0.04, the compounds which has a refractive index equal to orgreater than that, i.e. 1.56, of deuterated bromo benzene d-5. Examplesof the compounds satisfying these conditions are shown bellow, but notlimited to theses.

According to the first embodiment, the dopant is desirably selected fromthe group consisting of compounds which are denoted by the followingformula (1) and satisfy the above mentioned conditions.

Formula (1)

In the formula (1), R¹ to R¹⁰ respectively represent a hydrogen atom, analkyl, an alkenyl, an alkyloxy, an alkenyloxy, or dialkylamino providedthat at least four of them represent an alkyl, an alkenyl, an alkyloxy,an alkenyloxy or a dialkylamino.

One dopant or two or more dopants may be used in the polymerizablecomposition of the present invention. It is necessary that all compoundsused as a dopant are selected from the group consisting of the compoundshaving a benzene ring substituted by a substituent having a Hammettvalue not greater than 0.04, or by substituents having an average of theHammett values thereof not greater than 0.04.

An optical member having a graded refractive index can be prepared bygrading the concentration of the dopant while polymerization of thepolymerizable composition of the first embodiment. One usable processfor grading the dopant concentration is an interfacial gelpolymerization process described later.

Preferable ranges of the amount of the components respectively mayproperly be determined in consideration of species to be employed, wherethe additional amount of the polymerization initiator is desirablywithin a range from 0.005 to 0.5 wt % and more desirably within a rangefrom 0.010 to 0.50 wt %, with respect to of the polymerizable monomercomposition; and the additional amount of the chain transfer agent isdesirably within a range from 0.10 to 0.40 wt %, and more desirablywithin a range from 0.15 to 0.30 wt %, with respect to of thepolymerizable monomer composition. The additional amount of the dopat isdesirably in a range from 1 to 30 wt %, and more desirably in a rangefrom 1 to 25 wt %, with respect to the polymerizable monomercomposition.

It is noted that as the additional amount of the dopant is great, thethermoplasticity of a polymer being added the dopant is easilydeveloped, as with that the glass transition point, Tg, is lowered, andthe heat resistant property during use is lowered. Thus, the dopantwhich can achieve a desired distributed refractive index in smalleramount is preferred.

Another possible strategy relates to addition of other additives to thepolymerizable composition to an extent not degrading the lighttransmission property. For example, an additive can be added in order toimprove the weatherability or durability. It is also allowable to add anemission inductive material for amplifying light signal for the purposeof improving the light transmission property. Since even attenuatedlight signal can be amplified by addition of such compound to therebyelongate the length of transmission, the compound is typicallyapplicable to produce a fiber amplifier at a part of light transmissionlink.

When heat and/or light is irradiated to the polymerizable composition,radicals and the like are generated from the initiator, thereby inducingthe polymerization of the polymerizable monomer. Since the polymerizablecomposition according to the first embodiment contains the dopant, therefractive-index-distributed structure can readily be obtained bycontrolling the proceeding direction of the polymerization, typically bythe interfacial gel polymerization process described later, so as tocreate a concentration gradient of the dopant, or so as to create acopolymerization ratio gradient of the dopant and the polymerizablemonomer. According to the first embodiment, for preventing absorptionattributed to the fourth overtone of C—H stretching vibration in abenzene ring from influencing 850 nm light of a light source, the dopantimproved so that the absorption is significantly shifted to longerwavelengths is used, and therefore, transmission loss due to the dopantcan be lowered. The polymer having a desired molecular weight can beobtained by adjusting a polymerization rate and/or degree with apolymerization initiator or a chain transfer agent which is occasionallyadded to the composition. When the polymerizable composition comprisinga chain transfer agent is used, the molecular weight of the polymer canbe adjusted by the chain transfer agent so as to be suitable inmechanical properties for drawing. Therefore, using such composition canalso contribute to improvement in productivity when an optical fiber isprepared by drawing the preform produced by polymerization of thecomposition.

An optical member, which comprises a polymer composition and a dopantand has a distributed refractive index based on a distribution of thedopant concentration, can be produced by polymerization of thepolymerizable composition of the present invention. Since the dopant hasan absorption peak attributed to the fourth overtone of C—H stretchingvibration in a benzene ring at not greater than 875 nm, transmissionloss due to the dopant can be remarkably lowered. The dopant isdesirably selected from the group consisting of the above Formula (1).The polymer contained in the polymer composition is desirably selectedfrom the group consisting of homopolymers and copolymers of(meth)acrylates.

1-2 Second Embodiment of the Polymerizable Composition

The second embodiment of the polymerizable composition may be used forpreparing optical member for 850 nm light source wavelength. Thepolymerizable composition essentially comprises a polymerizable monomercomposition, a polymerization initiator capable of initiatingpolymerization thereof and a compound having a different refractiveindex from that of the monomer composition, occasionally referred to as“dopant” or “enhancer of refractive index” hereinafter. According to thesecond embodiment, by using a compound, having at least one benzene ringsubstituted by a substituent having a Hammett value within a particularrange or by substituents having an average value of Hammett valuesthereof within the particular range, as a dopant, transmission loss dueto the dopant can be lowered. In addition, under consideration ofcompatibility with a matrix material, by using a compound further havinga solubility parameter within a particular range, transmission loss ofthe optical member produced by polymerization of the composition can bereduced down to a further lower level. The polymerizable composition ofthe second embodiment may be used for producing optical members, inparticular optical members having a distribution in refractive indexvalues. Various materials used for the second embodiments are describedbellow.

1-2-1 Polymerizable Monomer Composition

According to the second embodiment, the polymerizable monomercomposition desirably comprises at least one selected from the groupconsisting of esters of propenoic acid and derivatives thereof ispreferably contained in major proportion. Examples of esters ofpropenoic acids and derivatives thereof include acrylates andmethacrylates, both of them are referred to as “(meth)acrylates”hereinafter. The term of “comprise a monomer in major proportion” isused for not only the embodiment consisting of the monomer, but alsoembodiments further comprising at least one polymerizable monomer otherthan the monomer so far as not lowering optical properties. For example,the composition may comprise at least one selected from monomers otherthan (meth)acrylates, such as styrene or maleimide, to form a copolymer.According to the second embodiment, in particular, in the viewpoint ofimprovement of moisture-heat-resistant property, alicyclic(meth)acrylates denoted by Formula (2) are used desirably.

Formula (2)

In the formula X¹ is hydrogen (H) or deuterium (D) wherein two X¹ may besame or different; Y¹ represents H, D, CH₃ or CD₃; and R¹ represents aC₇₋₂₀ alicyclic hydrocarbon group.

The polymerizable monomer denoted by the formula (2) is a (meth)acrylatederivative having a C₇₋₂₀ alicyclic hydrocarbon group. The examples ofthe polymerizable monomer include bicyclo [2.2.1]heptyl-2(meth)acrylate,1-adamantyl (meth)acrylate, 2-adamantyl (meth)acrylate,3-methyl-1-adamanthyl(meth)acrylate,3,5-dimethyl-1-adamantyl(meth)acrylate, 3-ethyladamanthyl(meth)acrylate,3-methyl-5-ethyl-1-adamanthyl(meth)acrylate,3,5,8-triethyl-1-adamanthyl(meth)acrylate,3,5-dimethyl-8-ethyl-1-adamanthyl(meth)acrylate,octahydro-4,7-menthanoindene-5-il(meth)acrylate,octahydro-4,7-menthanoidene-1-ylmethyl(meth)acrylate,tricyclodecyl(meth)acrylate,3-hydroxy-2,6,6-trimethyl-bicycl[3.1.1]heptyl(meth)acrylate,3,7,7-trimethyl-4-hydroxy-bicyclo[4.1.0 heptyl(meth)acrylate,(nor)bornyl(meth)acrylate, isobornyl(meth)acrylate,phentyl(meth)acrylate and 2,2,5-trimethylcyclohexyl(meth)acrylate. Amongthem, bornyl(meth)acrylate, isobornyl(meth)acrylate andphentyl(meth)acrylate are desirable, and bornyl(meth)acrylate andisobornyl(meth)acrylate are more desirable.

According to the second embodiment, two or more compounds selected fromthe group consisting of (meth)acrylates are desirably used as a majorcomponent in the polymerizable monomer composition. The content of atleast one compound denoted by the formula (2) is desirably from 5 to 95wt %, more desirably from 10 to 95 wt % and much more desirably from 10to 90 wt % with respect to the total weight of the polymerizable monomercomposition. In the viewpoint of compensating for brittleness andmechanistic properties of the compound denoted by the formula (2), otherpreferred examples of (meth)acrylates which can be used as apolymerizable monomer include methyl methacrylate, ethyl methacrylate,isopropyl methacrylate, t-butyl methacrylate, benzyl methacrylate,phenyl methacrylate, cyclohexyl methacrylate, methyl acrylate, ethylacrylate, n-butyl acrylate, t-butyl acrylate and phenyl acrylate. Amongthese, methyl methacrylate is most desirable.

When at least one compound selected from the group consisting of theformula (2) and methyl methacrylate as the other polymerizable monomerare used in combination, in order to ensure an adequate hydrohobicity,the content of the at least one compound selected from the groupconsisting of the formula (2) is desirably not lower than 10 wt % andmore desirably not lower than 15 wt % with respect to the total weightof the polymerizable monomers. When a polymerizable monomer other thanmethyl methacrylate is used, the preferred range is limited to therange.

At least one monomer other than (meth) acrylates may be used in thesecond embodiment. Other examples of the polymerizable monomer which canbe used in the second embodiment will be shown bellow, but not limitedto these.

(c) styrene base compounds such as styrene, alpha-styrene, chlorostyrene or bromo styrene;

(d) vinyl esters such as vinyl acetate, vinyl benzoate, vinylphenylacetate or vinyl chloroacetate; and

(e) maleimides such as N-n-butylmaleimide, N-t-butylmaleimide,N-isopropylmaleimide or N-cyclohexyl maleimide are exemplified.

According to the present invention, when at least two compounds selectedfrom the group consisting of (meth) acrylates are used as a majorcomponent of the polymerizable monomer composition, the total content ofthe (meth)acrylates is desirably not smaller than 50 wt %, moredesirably not smaller than 60 wt %, much more desirably not smaller than70 wt % and most desirably 100 wt % with respect to the total weight ofthe polymerizable monomer composition.

According to the second embodiment, as using a particular class of acompound described bellow are used for a dopant, the compositioncontaining the dopant has a greater refractive index than that of acomposition not containing the dopant, or the copolymer containing thedopant as a copolymerization monomer has a greater refractive index thanthat of a polymer not containing the dopant as a copolymerizationmonomer. The C—H bonds included in the polymerizable monomer maycontribute to increasing transmission loss of the optical member,especially for 850 nm light source wavelength, so that it is moredesirable to use deuterated polymerizable monomer including at least oneC-D bond.

1-2-2 Polymerization Initiator

The composition comprises a polymerization initiator which can initiatepolymerization of the polymerizable monomer composition. Thepolymerization initiator may be selected from known polymerizableinitiators depending on various factors such as polymerizable monomerscontained in the composition or the polymerization process. The examplesof the polymerization initiator include peroxides such as benzoylperoxide (BPO), t-butylperoxy-2-ethylhexanate (PBO), di-t-butylperoxide(PBD), t-butylperoxyisopropylcarbonate (PBI) orn-butyl-4,4-bis(t-butylperoxy)valerate (PHV); and azo compounds such as2,2′-azobisisobuthylonitrile, 2,2′-azobis(2-methylbuthylonitrile),1,1′-azobis(cyclohexane-1-carbonitrile), 2,2′-azobis(2-methylpropane),2,2′-azobis(2-methylbutane), 2,2′-azobis(2-methylpentane),2,2′-azobis(2,3-dimethylbutane), 2,2′-azobis(2-methylhexane),2,2′-azobis(2,4-dimethylpentane), 2,2′-azobis(2,3,3-trimethylbutane),2,2′-azobis(2,4,4-trimethylpentane), 3,3′-azobis(3-methylpentane),3,3′-azobis(3-methylhexane), 3,3′-azobis(3,4-dimethylpentane),3,3′-azobis(3-ethylpentane), dimethyl-2,2′-azobis(2-methylpropionate),diethyl-2,2′-azobis(2-methyl propionate) ordi-t-butyl-2,2′-azobis(2-methylpropionate). Two or more polymerizationinitiators may be used in combination.

1-2-3 Chain Transfer Agent

The composition according to the second embodiment may comprise a chaintransfer agent. The chain transfer agent may mainly be used foradjusting molecular weight of the obtained polymer. The chain transferagent can be properly selected in consideration of the monomer to beemployed. The chain transfer constants of the chain transfer agents forvarious monomers can be referred to publications such as “PolymerHandbook 3^(rd) edition” edited by J. BRANDRUP and E. H. IMMERGUT,published by JOHN WILEY&SON. The chain transfer constants can be alsoobtained by experimental tests according to methods disclosed in“Kohbunshi gousei no jikkenhou (Experimental methods for polymersynthesis)” written by Takayuki Ohtsu and Masaetsu Kinoshita, publishedby Kagaku-Dojin Publishing Company, INC (1972).

When methyl methacrylate is used as a polymerizable monomer, at leastone selected from the group consisting of alkylmercaptans(n-butylmercaptan, n-pentylmercaptan, n-octylmercaptan,n-laurylmercaptan, t-dodecylmercaptan, etc.) and thiophenols(thiophenol, m-bromothiophenol, p-bromothiophenol, m-toluenethiol,p-toluenethiol, etc.) is desirably used as a chian transfer agent. Amongthese, alkyl mercaptans such as n-octylmercaptan, n-laurylmercaptan ort-dodecylmercapton are preferred. It is possible to use the chaintransfer agents in which at least a part of hydrogen atoms of C—H bondsare replaced with deuterium atoms. Two or more chain transfer agents maybe used in combination.

1-2-4 Dopant: Enhancer of Refractive Index

The polymerizable composition according to the second embodimentcomprises a compound having a different refractive index from that ofthe polymerizable monomer composition. The dopant may be polymerizableor non-polymerizable. When a polymerizable dopant is used, it may bemore difficult to adjust various properties since the copolymerizationof the polymerizable monomer composition and the dopant may be carriedout, however, the advantage in heat resistant property may be obtained.The dopant is also referred to as enhancer of refractive index, and acompound having a property that increases the refractive index of acomposition containing it as compared with a composition not containingit, or increases the refractive index of a copolymer containing it as acopolymerization component as compared with a polymer not containing it.The difference in refractive index between the composition containingthe dopant and the composition not containing the dopant is desirablynot smaller than 0.001.

According to the second embodiment, at least one compound selected fromthe group consisting of benzene derivatives having a benzene ringsubstituted by at least one substituent is used as a dopant. The presentinventors found that there is a negative correlation between the Hammettvalue of the substituent and the wavelength of the absorption peakattributed to the fourth overtone of C—H stretching vibration in thebenzene ring. In other words, they found that the absorption peakattributed to the fourth overtone of C—H bond stretching vibration in abenzene ring is influenced by the substituents of other carbon atoms inthe benzene ring, and that the absorption peak attributed to the fourthovertone of the stretching vibration is shifted to longer wavelengths asthe Hammett values thereof are smaller. In order to lower transmissionloss at 850 nm, it is preferred that the absorption peak attributed tothe fourth overtone of C—H stretching vibration is shifted to longerwavelengths so that the absorption peak or the foot of the absorption isfar from 850 nm.

For preventing the dopant from influencing absorption at 850nm, thedopant desirably has the absorption peak at not shorter than 875 nm,more desirably at not shorter than 877 nm and much more desirably at notshorter than 880 nm. When the dopant having an absorption peak at notshorter than 880 nm is used, transmission loss may hardly occur due tothe absorption of a transmitting light of 850 nm.

In order to shift the absorption peak attributed to the fourth overtoneof C—H stretching vibration to longer wavelengths, the Hammett value ofa substituent or plural substituents is desirably not greater than 0.04,more desirably not greater than −0.05 and much more desirably notgreater than −0.1. The minimum of the Hammett value is desirably −0.6.In addition, by introduction of such a substituent or such substituents,the number of C—H bonds is decreased, and transmission loss due to anabsorption attributed fourth overtone of C—H stretching vibration itselfmay be further lowered as a secondary effect.

In the specification, when there is a substituent, the Hammett valuemeans the Hammett constant of the substituent as described in ChemicalReviews, Vol.91, No.2, pp.168-175(1991). The same Hammett constant isgenerally used for both of substituting at ortho position and at paraposition, on the other hand, the different Hammett constant is used forsubstituting at meta position. For example, given a benzene compoundsubstituted by a substituent, there are two hydrogen atoms respectivelyat ortho position and is one hydrogen atom at para position with respectto the position of the substituent R¹. When the Hammett constant of R¹at both of para and ortho positions is σ_(1p) and the Hammett constantof RI at meta position is σ_(1m), the Hammett value a can be calculatedwith the following formula:σ=(σ_(1p)×3+σ_(1m)×2)/5

On the other hand, when there are plural substituents, the Hammett valuemeans an average value of Hammett values thereof. The process forcalculating the Hammett value when there are plural substituents will bedescribed hereinafter with examples respectively having one benzene ringand two benzene rings. It is noted that the Hammett constant of R^(i),when i is any positive number, substituting at meta position is referredto as σ_(1p) and the Hammett constant of R^(i) substituting at eitherpara or ortho position is referred to as σ_(1p), in other words, theHammett constant for para position is used for ortho position.

Calculating method when there is one benzene ring:

In the above structure, the sum of the Hammett values of substituentsR¹, R² and R³ for each of the positions a, b and c is:σ_(a)=σ_(1p)+σ_(2m)+σ_(3m);σ_(b)=σ_(1m)+σ_(2p)+σ_(3p);σ_(c)=σ_(1m)+σ_(2p)+σ_(3p).

The average value of σ_(a), σ_(b) and σ_(c), i.e. the sum averagethereof, which is calculated according to the following formula, is theHammett value of the plural substituents which the above compound has.σ=(σ_(a)+σ_(b)+σ_(c))/3

Calculating method when there are plural substituents:

In the above structure, the sum of the Hammett values of substituentsR¹, R², R³ R⁴ and —S—Ar group for each of the positions a, b, c, d, eand f is:σ_(a)=σ_(1p)+σ_(2m)+σ_((Sph)m);σ_(b)=σ_(1m)+σ_(2p)+σ_((Sph)p);σ_(c)=σ_(1m)+σ_(2p)+σ_((Sph)p);σ_(d)=σ_(3m)+σ_(4p)+σ_((Sph)p);σ_(e)=σ_(3p)+σ_(4m)+σ_((Sph)m);σ_(f)=σ_(1m)+σ_(2p)+σ_((Sph)p).

It is noted that the Hammett constant of —S—Ph, i.e. σ_((Sph)m) orσ_((Sph)p), where Ph represents a non-substituted benzene ring, is usedfor —S—Ar, where Ar represents substituted phenyl, with regarding all—S—Ar as —S—Ph.

The average value of σ_(a) to σ_(f), i.e. the sum average thereof, whichis calculated according to the following formula, is the Hammett valueof the plural substituents that the above compound has.σ=σ_(a)+σ_(b)+σ_(c)+σ_(d)+σ_(e)+σ_(f))/6

Furthermore, the compatibility of the matrix material and the dopantused in combination also significantly influences to transmission loss.As described above, the matrix material having an adequate hydrohobicitycan have both of a good moisture-heat-resistant property and goodmechanical property. The present inventors conducted various studies toobtain dopants having a good compatibility, and as a result, they foundthat benzene derivatives which has a solubility parameter (SP) of notgreater than 10.9, desirably not greater than of 10.8, more desirablynot greater than 10.6, can exhibit a good compatibility, and by usagesuch compounds, optical members having good transparency and lowtransmission loss are obtainable. The examples of the dopant having abenzene ring substituted by one substituent or plural substituents,having a Hammett value of not greater that 0.04, and a SP value of notgreater than 10.9, are shown bellow, but not to limited to these. It isnoted that SP values were calculated according to Fedors' methoddescribed in “Polymer Engineering and Science”, vol. 14, P.147-154.

One dopant or two or more dopants may be used in the polymerizablecomposition of the second embodiment. It is necessary that all compoundsused as a dopant are selected from the group consisting of the compoundshaving a benzene ring substituted by a substituent group having aHammett value not greater than 0.04, or by substituents having anaverage of the Hammett values thereof not greater than 0.04 and a SPvalue of not greater than 10.9.

The desirable additional amount of the dopant may vary depending onincreasing ability of refractive index and interaction with the polymermatrix. In general, the additional amount of the dopant is desirablyfrom 1 to 30 wt %, more desirably from 3 to 25 wt % and much moredesirably from 5 to 20 wt % with respect to the total of thepolymerizable composition.

An optical member having a graded refractive index can be prepared bygrading the concentration of the dopant while polymerization of thepolymerizable composition of the first embodiment. One usable processfor grading the dopant concentration is an interfacial gelpolymerization process described later.

Preferable ranges of the amount of the components respectively mayproperly be determined in consideration of species to be employed, wherethe additional amount of the polymerization initiator is desirablywithin a range from 0.005 to 0.5 wt % and more desirably within a rangefrom 0.010 to 0.50 wt % with respect to of the polymerizable monomercomposition, and the additional amount of the chain transfer agent isdesirably within a range from 0.10 to 0.40 wt % and more desirablywithin a range from 0.15 to 0.30 wt % of the monomer composition. Theadditional amount of the agent is desirably in a range from 1 to 30 wt %of the polymerizable monomer composition, and more desirably in a rangefrom 1 to 25 wt %.

Another possible strategy relates to addition of other additives to thepolymerizable composition to an extent not degrading the lighttransmission property. For example, an additive can be added in order toimprove the weatherability or durability. It is also allowable to add anemission inductive material for amplifying light signal for the purposeof improving the light transmission property. Since even attenuatedlight signal can be amplified by addition of such compound to therebyelongate the length of transmission, the compound is typicallyapplicable to produce a fiber amplifier at a part of light transmissionlink.

When heat and/or light is irradiated to the polymerizable composition,radicals and the like are generated from the polymerizable initiator,thereby inducing the polymerization of at least one polymerizablemonomer. Since the polymerizable composition according to the secondembodiment contains the dopant, the refractive-index-graded structurecan readily be obtained by controlling the proceeding direction of thepolymerization, typically by the interfacial gel polymerization processdescribed later, so as to create a concentration gradient of the dopant,or so as to create a copolymerization ratio gradient of the dopant andthe at least one polymerizable monomer. According to the secondembodiment, for preventing absorption attributed to the fourth overtoneof C—H stretching vibration in a benzene ring from influencing 850 nmlight of a light source, the dopant improved so that the absorption issignificantly shifted to longer wavelengths is used, and therefore,transmission loss due to the dopant can be lowered. Furthermore, fromthe viewpoint of the compatibility with the polymer matrix having anadequate hydrohobicity, by using the dopant having a SP value within aspecific range, transmission loss is further lowered andmoisture-heat-resistant property is improved. The polymer having adesired molecular weight can be obtained by adjusting a polymerizationrate and/or degree with a polymerization initiator or a chain transferagent which is occasionally added to the composition. When thepolymerizable composition comprising a chain transfer agent is used, themolecular weight of the polymer can be adjusted by the chain transferagent so as to be suitable in mechanical properties for drawing.Therefore, using such composition can also contribute to improvement inproductivity when an optical fiber is prepared by drawing the preformproduced by polymerization of the composition.

2. Optical Member

Examples processes for producing optical members with the polymerizablecomposition of the first or second embodiment will be described indetail. The examples described bellow are the examples in which thepolymerizable composition of the first or second embodiment is used forproducing a core region of graded-refractive-index optical membercomprising the core region and a clad region.

GI type optical members can be produced by a process comprising a firststep of preparing a hollow structure (for example a cylinder)corresponding to the clad region by carrying out polymerization of apolymerizable composition; a second step of preparing a preform whichcomprises regions respectively corresponding to the core region and theclad region by carrying out polymerization of a polymerizablecomposition of the first or second embodiment in the hollow portion ofthe structure; and a third step of processing the obtained preform intovarious forms.

A hollow structure (for example cylinder) made of a polymer is obtainedthrough the first step. As typically described in International PatentPublication WO93/08488, a polymerizable composition is poured into acylindrical polymerization vessel, and then polymerization is carriedout while rotating (preferably while keeping the axis of the cylinderhorizontally) the vessel, referred to as “a rotational polymerization”herein after, to thereby form a cylinder made of a polymer. At least onepolymerization initiator, at least one transfer agent and if necessary,at least one additive such as a stabilizer can be poured into the vesselwith at least polymerizable monomer. A suitable temperature and periodfor the polymerization may vary depending on species of the monomers tobe employed. In general, the polymerization is preferably carried out at60 to 90 degrees Celsius for 5 to 24 hours. The monomers used herein maybe pre-polymerized before the polymerization so as to raise theviscosity thereof as described in JPA No. 1996-110419 (the term “JPA” asused herein means an “unexamined published Japanese patent application).Since the obtained hollow structure may be deformative when the vesselmay get distorted by rotation, it is preferable to use a metal or glassvessel having a sufficient rigidity.

The major ingredient of the polymerizable monomer composition used inthe first step is desirably same as that of the polymerizable monomercomposition used in the second step. The ration and the minor ingredientthereof may be same or different.

When the polymerizable composition of the first embodiment is used inthe second step, the usable polymerizable monomers used in the firststep for forming the clad region are not specifically limited. Examplesthereof include (meth) acrylates such as methyl methacrylate, t-butylmethacrylate, cyclohexyl methacrylate, tricyclo [5.2.1.0^(2,6)]decanylmethacrylate, adamantyl methacrylate, isobornyl methacrylate; andfluorinated (meth)acrylates such as 2,2,2-trifluoroethyl methacrylate,2,2,3,3-tetrafluoropropyl methacrylate, 2,2,3,3,3,-prntafluoropropylmethacrylate, 1-trifluoromethyl-2,2,2-trifluoroethyl methacrylate,2,2,3,3,4,4,5,5-octafluoropentyl methacrylate or2,2,3,3,4,4-hexafluorobutyl methacrylate. One polymerizable monomer ortwo or more polymerizable monomers may be used in the first step. Whentwo or more polymerizable monomers are used, the desirable ratio thereofmay vary depending on types of the polymerizable monomers.

When the polymerizable composition of the second embodiment is used inthe second step, the usable polymerizable monomers used in the firststep for forming the clad region are not specifically limited. Usableexamples thereof are same as those exemplified above. At least onecompound selected from the group consisting of the formula (2) isdesirably used in the first step.

Another material such as a polymerization initiator or a chain transferagent used in the first step is not specifically limited, and may beselected from the various known materials depending on the polymerizablemonomer used. The examples of such material are same as thoseexemplified for the first or second polymerizable compositions. Ingeneral, the desirable additional amount of the polymerization initiatormay be in a range of 0.1 to 1.00 wt %, more desirably in a range of 0.40to 0.60 wt %, of the monomer; and the desirable additional amount of thechain transfer agent may be in a range of 0.10 to 0.40 wt %, moredesirably in a range of 0.15 to 0.30wt %, of themonomer.

The hollow cylinder desirably has a bottom portion, so as that amaterial for the core region can be poured into the cylinder in thesecond step. The preferred material for the bottom portion is a materialhaving a good affinity and adhesiveness with the polymer of thecylinder. The bottom portion may be formed of the same polymer as thatof the cylinder. For example, the bottom potion can be produced bypouring a small amount of monomer into a vessel before or after carryingout rotational polymerization; and carrying out polymerization of themonomer with still standing the vessel.

For the purpose of completely reaction of the residual monomer or theresidual polymerization initiator, it is allowable after such rotationalpolymerization to carry out annealing at a temperature higher than thepolymerization temperature, or to remove non-polymerized components.

In the first step, it is also possible to produce the structure having adesired shape (cylindrical shape in this embodiment) by molding apolymer using known molding technique such as extrusion molding.

When the cylindrical structure is produced by molding and thepolymerizable composition of the second embodiment is used in the secondstep, the examples of the polymers which can be used include polymersproduced by polymerization of the second polymerizable composition, butnot containing any dopants; polyvinylidene fluorides, fluorinated(meth)acrylate base polymers and amorphous polyolefins. The fluorinatedpolymers generally have a refractive index lower than that of thenon-fluorinated polymers, and using such fluorinated polymers, thenumerical aperture number is increased; however, the adhesion to thecore region is sometimes lowered. In such case, to ensure the sufficientadhesion, an outer core layer may be formed between the core and cladregions, by polymerization of the second polymerizable composition, butnot containing any dopants. The clad region and the outer core regionmay be produced at same time by molding techniques such as co-extrusionmolding.

In the second step, the polymerizable composition of the first or secondembodiment is poured into the hollow portion of the cylinder, which wasobtained by the first step, corresponding to the clad region, and thepolymerization of the monomer is carried out. When the outer core regionwas formed, the polymerizable monomer composition was poured into thehollow portion of the two-layered cylinder. From the view point ofresidues after polymerization, it is preferred to carry out thepolymerization by a method based on the interfacial gel polymerizationprocess which is solvent-free, disclosed in International PatentPublication No. WO93/08488. In the interfacial gel polymerizationprocess, the polymerization of the polymerizable monomer proceeds alongthe radial direction of the cylinder from the inner wall thereof, ofwhich viscosity is high, towards the center due to gel effect. When thepolymerizable composition containing the dopant is used in thepolymerization, the polymerization proceeds in a way such that themonomer having a higher affinity to the polymer of the cylinderpredominantly exists on the inner wall of the cylinder and thenpolymerizes, so as to produce on the outer periphery a polymer having alower content of the dopant. Ratio of the dopant in the resultantpolymer increases towards the center. This successfully creates thedistribution of the concentration of the dopant and thus introduces thedistribution of refractive index within the region corresponding to acore region. And, when the dopant has a polymerizable group, the monomerhaving a higher affinity to the polymer of the cylinder predominantlyexits on the inner wall of the cylinder and then polymerizes, so as toform a polymer having a low copolymerization ration of the dopant in theoutside. The copolymerization ratio of the dopant in the resultantpolymer increases towards the center. This successfully creates thedistribution of the copolymerization ratio of the dopant, and thusintroduces the distribution of refractive index within the region basedon the graded copolymerization ratio corresponding to a core region.

Not only the distribution of refractive index is induced into the areacorresponding to the core region through the second step, but also thedistribution of thermal behavior since the areas having differentrefractive indices are also different in the thermal behavior. If thepolymerization in the second step is carried out at a constanttemperature, the response property against the volume shrinkage whichoccurs in the polymerization reaction process may vary depending on thethermal behaviors, and thereby air bubbles or micro-gaps may generate inthe obtained preform, and drawing under heating of such preform mayresult in that the obtained fiber has a lot of air bubbles formedtherein. If the polymerization in the second step is carried out at toolow temperature, the productivity may considerably lower due to lowpolymerization efficiency, or the light transmission performance of theresultant optical member may lower due to incomplete polymerization. Onthe contrary, if the polymerization in the second step is carried out attoo high initial polymerization temperature, the initial polymerizationrate may be so fast that the volume shrinkage of the core region cannotbe reduced by a relaxation response, and as a result a lot of airbubbles may generate in the core region. Therefore, it is preferable tocarry out the polymerization at a proper temperature. When typicalmethacrylic esters are used as the monomer, the polymerization isdesirably carried out at a temperature within a range from 50 to 150degrees Celsius, more desirably at a temperature within a range from 80to 120 degrees Celsius. It is also preferable to carry out thepolymerization under inert gas atmosphere applied pressure in order toimprove response property against the volume shrinkage which occurs inthe polymerization. By dehydration or deaeration, it is possible tofurther reduce the degree of generating air bubbles.

(1) Using the First Polymerizable Composition:

The desirable polymerization temperature and polymerizable time may varywith the polymerizable monomers used. In general, the polymerizationtemperature is desirably from 60 to 90 degrees Celsius and thepolymerization time is desirably from 5 to 24 hours. It is alsodesirable that a polymerization initiator having a ten-hour half-timedecomposition temperature not lower than the boiling point of apolymerizable monomer is used, and the polymerization of thepolymerizable monomer is carried out for 25 % of the half-life period ofthe polymerization initiator. When the polymerization is carried outsuch conditions, it is possible to lower the initial polymerizationrate, to improve the response property against the volume shrinkage, andthus, to reduce the degree of generating air bubbles in the preform dueto the volume shrinkage and to increase the productivity. It is to benoted now that ten-hour, half-life decomposition temperature of thepolymerization initiator means a temperature at that the polymerizationinitiator decomposes and reduces to the half amount for ten hours. Whenmethyl methacrylate (MMA) is used as the polymerizable monomer,2,2′-azobis(2-methylpropane) or 2,2′-azobis(2,4,4-trimethylpentane) canbe selected from the above-listed polymerization initiators such thathaving ten-hour, half-life decomposition temperature not lower than theboiling point of MMA. When MMA is used as the polymerizable monomer andthe later is used as the polymerization initiator, the polymerization ispreferably allowed to proceed while keeping the initial polymerizationtemperature at 100 to 110 degrees Celsius for 48 to 72 hours, andfurther allowed to proceed at a temperature elevated to 120 to 140degrees Celsius for 24 to 48 hours. When the former is used as thepolymerization initiator, the polymerization is preferably allowed toproceed while keeping the initial polymerization temperature at 100 to110 degrees Celsius for 4 to 24 hours, and further allowed to proceed ata temperature elevated to 120 to 140 degrees Celsius for 24 to 48 hours.The temperature elevation may be effected either in a step-wise manneror in a continuous manner, where shorter time for the elevation ispreferable.

(2) When the Second Polymerizable Composition is used:

The desirable conditions for polymerization of the second polymerizablecomposition containing at least alicyclic (meth)acrylate base monomerdenoted by the formula (2) are same as those for polymerization of apolymerizable composition containing a typical (meth)acrylate.Therefore, the polymerization is desirably carried out at a temperaturewithin a range from 50 to 150 degrees Celsius, more desirably at atemperature within a range from 80 to 120 degrees Celsius. It is alsopreferable to carry out the polymerization under inert gas atmosphereapplied pressure in order to improve response property against thevolume shrinkage which occurs in the polymerization. The polymerizationis preferably allowed to proceed while keeping the initialpolymerization temperature at 100 to 110 degrees Celsius for 4 to 24hours, and further allowed to proceed at a temperature elevated to 120to 140 degrees Celsius for 24 to 48 hours. Polymerization initiator maybe selected depending on the polymerization temperature orpolymerization time. When polymerization is carried out in the abovementioned condition, polymerization initiator is desirably selected fromthe group consisting of high-temperature decomposition type initiatorssuch as di-tert-butyl peroxide (PBD) or2,2′-azobis(2,4,4-trimethylpentane). The temperature elevation may beeffected either in a step-wise manner or in a continuous manner, whereshorter time for the elevation is preferable.

In the second step, it is preferable to carry out the polymerizationunder pressure (herein after referred as “pressurized polymerization”).In case of the pressurized polymerization, it is preferable to place thecylinder in the hollow space of a jig, and to carry out thepolymerization while keeping the cylinder as being supported by the jig.While the pressurized polymerization is being carried out in a hollowportion of the structure corresponding to the clad region, the structureis kept as being inserted in the hollow space of the jig, and the jigprevents the shape of the structure from being deformed due to pressure.The jig is preferably shaped as having a hollow space in which thestructure can be inserted, and the hollow space preferably has a profilesimilar to that of the structure. Since the structure corresponding tothe clad region is formed in a cylindrical form in the presentembodiment, it is preferable that also the jig has a cylindrical form.The jig can suppress deformation of the cylinder during the pressurizedpolymerization, and supports the cylinder so as to relax the shrinkageof the area corresponding to the core region with the progress of thepressurized polymerization. It is preferable that the jig has a hollowspace having a diameter larger than the outer diameter of themono-layered or double layered cylinder, and that the jig supports thecylinder corresponding to the clad region in a non-adhered manner. Sincethe jig has a cylindrical form in the present embodiment, the innerdiameter of the jig is preferably larger by 0.1 to 40% than the outerdiameter of the cylinder corresponding to the clad region, and morepreferably larger by 10 to 20%.

The cylinder can be placed in a polymerization vessel while beinginserted in the hollow space of the jig. In the polymerization vessel,it is preferable that the cylinder is housed so as to vertically alignthe height-wise direction thereof. After the cylinder is placed, whilebeing supported by the jig, in the polymerization vessel, thepolymerization vessel is pressurized. The pressurizing of thepolymerization vessel is preferably carried out using an inert gas suchas nitrogen, and thus the pressurized polymerization preferably iscarried out under an inert gas atmosphere. While a preferable range ofthe pressure during the polymerization may vary with species of themonomer, it is generally 0.05 to 1.0 MPa or around.

A preform for the plastic optical member can be obtained through thefirst and second steps.

In the third step, a desired optical member can be obtained byprocessing the preform produced through above steps. For example,slicing the preform gives plate-shaped or column-shaped planar lens, anddrawing under fusion gives plastic optical fiber.

Optical fibers can be produced by heat drawing in the third step. Whilethe heating temperature during the drawing may properly be determined inconsideration of source material of the preform, a generally preferablerange thereof is 180 to 250 degrees Celsius. Conditions for the drawing(drawing temperature, etc.) may properly be determined in considerationof diameter of the obtained preform, desirable diameter of the plasticoptical fiber, and source materials used. In particular for the opticalfiber having a graded refractive index, the drawing spinning and heatingshould be carried out uniformly so as not to ruin the distributionprofile of the refractive index which varies along the radial direction.It is therefore preferable to heat the preform using a cylindricalheating oven capable of uniformly heating it in the sectional directionthereof, and to draw the preform into fiber using a draw-spinningapparatus which has an aligning mechanism for keeping the centerposition constant. The drawing tension can be set to 10 g or above inorder to orient molten plastic as described in JPA No. 1995-234322, andpreferably set to 100 g or below so that strain does not remain afterthe spinning as disclosed in JPA No. 1995-234324. It is also allowableto employ a method having a pre-heating step prior to the drawing.

The plastic optical fiber after being processed in the third step candirectly be subjected, without any modification, to variousapplications. The fiber may also be subjected to various applications ina form of having on the outer surface thereof a covering layer orfibrous layer, and/or in a form having a plurality of fibers bundled forthe purpose of protection or reinforcement.

For the case where a coating is provided to the element wire, thecovering process is such that running the element wire through a pair ofopposing dies which has a through-hole for passing the element fiber,filling a molten polymer for the coating between the opposing dies, andmoving the element fiber between the dies. The covering layer ispreferably not fused with the element fiber in view of preventing theinner element fiber from being stressed by bending. In the coveringprocess, the element fiber may be thermally damaged typically throughcontacting with the molten polymer. It is therefore preferable to setthe moving speed of the element fiber so as to minimize the thermaldamage, and to select a polymer for forming the covering layer which canbe melted at a low temperature range. The thickness of the coveringlayer can be adjusted in consideration of fusing temperature of polymerfor forming the covering layer, drawing speed of the element fiber, andcooling temperature of the covering layer.

Other known methods for forming the covering layer on the fiber includea method by which a monomer coated on the optical member is polymerized,a method of winding a sheet around, and a method of passing the opticalmember into a hollow pipe obtained by extrusion molding.

Coverage of the element fiber enables preparing of plastic optical fibercable. Styles of the coverage include contact coverage in which plasticoptical fiber is covered with a cover material so that the boundary ofthe both comes into close contact over the entire circumference; andloose coverage having a gap at the boundary of the cover material andplastic optical fiber. The contact coverage is generally preferablesince the loose coverage tends to allow water to enter into the gap fromthe end of the cover layer when a part of the cover layer is peeled offtypically at the coupling region with a connector, and to diffuse alongthe longitudinal direction thereof. The loose coverage in which thecoverage and element fiber are not brought into close contact, however,is preferably used in some purposes since the cover layer can relievemost of damages such as stress or heat applied to the cable, and canthus reduce damages given on the element fiber. The diffusion of waterfrom the end plane is avoidable by filling the gap with a fluidgel-form, semi-solid or powdery material. The coverage with higherperformance will be obtained if the semi-solid or powdery material isprovided with functions other than water diffusion preventive function,such as those improving heat resistance, mechanical properties and thelike.

The loose coverage can be obtained by adjusting position of a headnipple of a crosshead die, and by controlling a decompression device soas to form the gap layer. The thickness of the gap layer can be adjustedby controlling the thickness of the nipple, or compressing/decompressingthe gap layer.

It is further allowable to provide another cover layer (secondary coverlayer) so as to surround the existing cover layer (primary cover layer).The secondary cover layer may be added with flame retarder, UV absorber,antioxidant, radical trapping agent, lubricant and so forth, which maybe included also in the primary cover layer so far as a satisfactorylevel of the anti-moisture-permeability is ensured.

While there are known resins or additives containing bromine or otherhalogen or phosphorus as the flame retarder, those containing metalhydroxide are becoming a mainstream from the viewpoint of safety such asreduction in emission of toxic gas. The metal hydroxide has crystalwater in the structure thereof and this makes it impossible tocompletely remove the adhered water in the production process, so thatthe flame-retardant coverage is preferably provided as an outer coverlayer (secondary cover layer) surrounding the anti-moisture-permeabilitylayer (primary cover layer) of the present invention.

It is still also allowable to stack cover layers having a plurality offunctions. For example, besides flame retardation, it is allowable toprovide a barrier layer for blocking moisture absorption by the elementfiber or moisture absorbent for removing water, which is typified byhygroscopic tape or hygroscopic gel, within or between the cover layers.It is still also allowable to provide a flexible material layer forreleasing stress under bending, a buffer material such as foaming layer,and a reinforcing layer for raising rigidity, all of which may beselected by purposes. Besides resin, a highly-elastic fiber (so-calledtensile strength fiber) and/or a wire material such as highly-rigidmetal wire are preferably added as a structural material to athermoplastic resin, which reinforces the mechanical strength of theobtained cable.

Examples of the tensile strength fiber include aramid fiber, polyesterfiber and polyamide fiber. Examples of the metal wire include stainlesswire, zinc alloy wire and copper wire. Both of which are by no meanslimited to those described in the above. Any other protective armor suchas metal tube, subsidiary wire for aerial cabling, and mechanisms forimproving workability during wiring can be incorporated.

Types of the cable include collective cable having element fibersconcentrically bundled; so-called tape conductor having element fiberslinearly aligned therein; and collective cable further bundling them bypress winding or wrapping sheath; all which can be properly selecteddepending on applications.

The optical member of the present invention is available as an opticalfiber cable for use in a system for transmitting light signal, whichsystem comprises various light-emitting element, light-receivingelement, other optical fiber, optical bus, optical star coupler, lightsignal processing device, optical connector for connection and so forth.Any known technologies may be applicable while making reference to“Purasuchikku Oputicaru Faiba no Kiso to Jissai (Basics and Practice ofPlastic Optical Fiber)”, published by N.T.S. Co., Ltd.; optical bustypically described in JPA Nos. hei 10-123350, 2002-90571 and2001-290055; optical branching/coupling device typically described inJPA Nos. 2001-74971, 2000-329962, 2001-74966, 2001-74968, 2001-318263and 2001-311840; optical star coupler typically described in JPA No.2000-241655; light signal transmission device and optical data bussystem typically described in JPA Nos. 2002-62457, 2002-101044 and2001-305395; light signal processor typically described in JPA No.2002-23011; light signal cross-connection system typically described inJPA No. 2001-86537; optical transmission system typically described inJPA No. 2002-26815; and multi-function system typically described in JPANos. 2001-339554 and 2001-339555.

Outside of the above mentioned applications, the optical member of thepresent invention may be used in the various technical fields such aslighting systems, energy transmitters, illuminations or sensors.

EXAMPLES

The present invention will specifically be described referring to thespecific examples. It is to be noted that any materials, reagents, ratioof use, operations and so forth can properly be altered withoutdeparting from the spirit of the present invention. The scope of thepresent invention is therefore by no means limited to the specificexamples shown below.

Example 1-1

An amount of a mixture containing deuterated MMA (MMA-d8) as apolymerizable monomer, which was removed hydroquinone monomethyl etheras a polymerization inhibitor and reduced water content by not greaterthan 80 ppm, 0.5 wt %, with respect to the monomer weight, of benzoylperoxide (BPO) as a polymerization initiator, and 0.28 wt %, withrespect to the monomer weight, of n-laurylmercaptan as a chain transferagent, was poured into a sufficiently-rigid cylindrical vessel having 22mm in inner diameter and 600 mm in length, which inner diametercorresponds with the outer diameter of the preform to be obtained. Thevessel was placed in the water bath at 80 degrees Celsius and themixture was shaken and pre-polymerized at 80 degrees Celsius for 2hours. Subsequently, the mixture was allowed to polymerize under heatingat 80 degrees Celsius for 3 hours while holding the vessel horizontallyand rotating it at a speed of rotation of 3, 000 rpm, which was followedby annealing at 100 degrees Celsius for 24 hours, to thereby obtainhollow cylinder made of the polymer of MMA-d8.

Next, MMA-d8 as a polymerizable monomer, which was removed hydroquinonemonomethyl ether as a polymerization inhibitor and was reduced watercontent by not greater than 80 ppm, and 10 wt %, with respect to themonomer weight, of a compound (13), (16) or (28) described above, or acompound (34), (35) or (36) as a comparative example described bellow,were mixed. The mixed solution was directly poured into the hollowregion of the obtained hollow cylinder while being filtered through amembrane filter, based on tetrafluoroethylene, having a pore size of 0.2μm. 0.016 wt %, with respect to the monomer weight, of PDB as apolymerization initiator and 0.27 wt %, with respect to the monomermixture weight, of n-laurylmercaptan as a chain transfer agent wereadded to the mixed solution. The chain transfer constant ofn-laurylmercaptan in this system was 0.8. A cylinder poured the mixedsolution into was housed in a glass tube having a diameter larger by 9%than the outer diameter of the cylinder, and was then allowed to standvertically in a pressure polymerization reactor. The inner atmosphere ofthe pressure polymerization reactor was then purged with nitrogen,pressurized up to 0.6 MPa, and the heat polymerization was allowed toproceed at 100 degrees Celsius for 48 hours and subsequently 120 degreesCelsius for 24 hours with keeping the pressured atmosphere to therebyobtain the preform.

The obtained preform observed when the polymerization completed wasfound to have no air bubbles contained therein which possibly introducedby volume shrinkage. The preform was drawn by thermal drawing at 230degrees Celsius so as to form a plastic optical fiber having a diameterof approx. 700 to 800 μm. The preform was not found to include airbubbles during the drawing, which contributed to successfully obtain thefiber of 300 m long in a stable manner.

The transmission loss for 850 nm light source and transmission band ofeach obtained optical fibers were shown in Table 1-1 with the Hammettvalue and the wavelength of the absorption peak attributed to the fourthovertone of benzene ring C—H stretching vibration of the dopant used ineach optical fibers.

It is noted that the Hammett value of the compound (35) is a value whenall C-D bonds are regarded as C—H bond, and since the wavelength of theabsorption peak attributed to the fourth overtone of C—H stretchingvibration can be substantially ignored at a deuteration rate of notlower than 99.5%, the Hammett value of the compound (35) is not shown inthe Table 1-1.

To investigate Tg decrease due to these dopants, 10 wt % of each thesedopants was added to MMA-d8, bulk polymerization of the MMA-d8 wascarried out, and the Tg of each obtained bulk polymers was measured. Theobtained Tg values are shown in Table 1-2.

Example 1-2

A mixture of MMA-d8, which was removed hydroquinone monomethyl ether asa polymerization inhibitor and reduced water content by not greater than80 ppm as the above, and a fluorinated deuterated monomer, 3FM-d7 shownbellow, in which the weight ratio of the former to the later was 9:1,was used as a polymerizable monomer. The 8 wt % of a same dopant as usedin the Example 1-1, i.e. compound (13), (16), (28), (35) or (36), wasadded to the monomer mixture. Except theses, optical fibers wereproduced in the same manner as the Example 1-1.

The obtained preform observed when the polymerization completed wasfound to have no air bubbles contained therein which possibly introducedby volume shrinkage. The preform was drawn by thermal drawing at 230degrees Celsius so as to form a plastic optical fiber having a diameterof approx. 700 to 800 μm. The preform was not found to include airbubbles during the drawing, which contributed to successfully obtain thefiber of 300 m long in a stable manner.

The transmission loss for 850 nm light source and transmission band ofeach obtained optical fibers were shown in Table 1-3 with the Hammettvalue of the dopant used in each optical fibers.

Example 1-3

A mixture of MMA-d8, which was removed hydroquinone monomethyl ether asa polymerization inhibitor and reduced water content by not greater than80 ppm as the above, and a deuterated monomer, tBMA-d14 shown bellow, inwhich the weight ratio of the former to the later was 1:1, was used as apolymerizable monomer. The 10 wt % of a same dopant as used in theExample 1-1, i.e. compound (13), (16), (28), (35) or (36), was added tothe monomer mixture. Except theses, optical fibers were produced in thesame manner as the Example 1-1.

The obtained preform observed when the polymerization completed wasfound to have no air bubbles contained therein which possibly introducedby volume shrinkage. The preform was drawn by thermal drawing at 230degrees Celsius so as to form a plastic optical fiber having a diameterof approx. 700 to 800 μm. The preform was not found to include airbubbles during the drawing, which contributed to successfully obtain thefiber of 300 m long in a stable manner.

The transmission loss for 850 nm light source and transmission band ofeach obtained optical fibers were shown in Table 1-4 with the Hammettvalue of the dopant used in each optical fibers. TABLE 1-1 TransmissionTransmission Wave- Hammett loss band length* Monomer Dopant value[dB/km] [GHz/km] [nm] MMA-d8 (13) −0.073  99 1.0 880 MMA-d8 (16) −0.28 97 1.0 883 MMA-d8 (28) −0.17 102 1.0 882 MMA-d8 (34) 0.134 400 1.0 873MMA-d8 (35) (0.294) 100 0.7 — MMA-d8 (36) 0.578 720 0.7 867*The wavelength of the absorption peak attributed to the fourth overtoneof benzene ring C—H stretching vibration

fourth overtone of benzene ring C—H stretching vibration TABLE 1-2 Tg**Monomer Dopant [° C.] MMA-d8 (13) 89 MMA-d8 (16) 90 MMA-d8 (28) 90MMA-d8 (34) 82 MMA-d8 (35) 81 MMA-d8 (36) 87**Tg of the polymer containing 10 wt % of the dopant

TABLE 1-3 Transmission loss Monomer Dopant Hammett value [dB/km]MMA-d8/3FM-d7 (13) −0.073  98 (9:1) MMA-d8/3FM-d7 (16) −0.28 102 (9:1)MMA-d8/3FM-d7 (28) −0.17 100 (9:1) MMA-d8/3FM-d7 (34) 0.134 405 (9:1)MMA-d8/3FM-d7 (35) — 101 (9:1) MMA-d8/3FM-d7 (36) 0.578 751 (9:1)

TABLE 1-4 Transmission Loss Monomer Dopant Hammett value [dB/km]MMA-d8/tBMA-d14 (13) −0.073 101 (1:1) MMA-d8/tBMA-d14 (16) −0.28  99(1:1) MMA-d8/tBMA-d14 (28) −0.17  99 (1:1) MMA-d8/tBMA-d14 (34) 0.134402 (1:1) MMA-d8/tBMA-d14 (35) —  98 (1:1) MMA-d8/tBMA-d14 (36) 0.578710 (1:1)

As shown in the Tables 1 to 4, it was found that when the compound (34)or (36), having the Hammett value greater than the specific value, i.e.0.04, was used, the transmission loss at 850 nm was much greater thanthat of when the compound (35) was used; and when the compound (13),(16) or (28), having the Hammett value not greater than 0.04, was used,the optical fiber had a same level of that of the obtained optical fiberwhen the compound (35) was used.

It was also found that when the dopant of the present invention wasused, in comparison with when the compound (35) was used, the broadertransmission band was obtained with the same additional amount and thebetter heat resistant property was obtained due to the higher Tg of thepolymer.

Example 2-1 and 2-2 and Comparative Example 2-1

An amount of a monomer mixture of deuterated methyl methacrylate,MMA-d8, and isobornyl methacrylate, IBXMA, both of them being removedhydroquinone monomethyl ether as a polymerization inhibitor and reducedwater content by 80 ppm, and the MMA-d8 to IBXMA weight ratio being 4/1,was poured into a sufficiently-rigid cylindrical vessel having 22 mm ininner diameter and 600 mm in length, which inner diameter correspondswith the outer diameter of the preform to be obtained. And 0.5 wt %,with respect to the monomer mixture weight, of benzoyl peroxide (BPO) asa polymerization initiator and 0.28 wt %, with respect to the monomermixture weight, of n-laurylmercaptan as a chain transfer agent wereadded to the monomer mixture. The vessel was placed in the water bath at80 degrees Celsius and the mixture was shaken and pre-polymerized at 80degrees Celsius for 2 hours. Subsequently, the mixture was allowed topolymerize under heating at 80 degrees Celsius for 3 hours while holdingthe vessel horizontally and rotating it at a speed of rotation of 3,000rpm, which was followed by annealing at 100 degrees Celsius for 24 hoursto thereby obtain hollow cylinder made of the copolymer of MMA-d8 andIBXMA.

Next, a monomer mixture of MMA-d8 and IBXMA, both of them being removedhydroquinone monomethyl ether as a polymerization inhibitor and reducedwater content by not greater than 80 ppm, and the MMA-d8 to IBXMA weightratio being 4/1; and 12.5 wt %, with respect to the monomer mixtureweight, of a compound 2-(4) or 2-(6) described above or a comparativecompound 2-(19) described bellow as a dopant were mixed, thereby toobtain a mixed solution. The mixed solution was directly poured into thehollow region of the obtained hollow cylinder while being filteredthrough a membrane filter, based on tetrafluoroethylene, having a poresize of 0.2 μm. 0.016 wt %, with respect to the monomer mixture weight,of PBD as a polymerization initiator and 0.27 wt %, with respect to themonomer mixture weight, of n-laurylmercaptan as a chain transfer agentwere added to the mixed solution. The chain transfer constant ofn-laurylmercaptan in this system was 0.8. A cylinder poured the mixedsolution into was housed in a glass tube having a diameter larger by 9%than the outer diameter of the cylinder, and was then allowed to standvertically in a pressure polymerization reactor. The inner atmosphere ofthe pressure polymerization reactor was then purged with nitrogen,pressurized up to 0.2 MPa, and the heat polymerization was allowed toproceed at 100 degrees Celsius for 48 hours and subsequently 120 degreesCelsius for 24 hours with keeping the pressured atmosphere to therebyobtain the preform.

Comparative Compound 2-(19)

The obtained preform observed when the polymerization completed wasfound to have no air bubbles contained therein which possibly introducedby volume shrinkage. The preform was drawn by thermal drawing at 230degrees Celsius so as to form a plastic optical fiber having a diameterof approx. 700 to 800 μm. The preform was not found to include airbubbles during the drawing, which contributed to successfully obtain thefiber of 300 m long in a stable manner.

The transmission loss for 850 nm light source of each obtained opticalfibers was measured. And after being left under condition of that thetemperature was 25 degrees Celsius and RH was 95%, the transmission lossfor 850 nm light source of each obtained optical fibers was measured.The increase of transmission loss was shown in Table 2-1.

Example 2-3 and 2-4 and Comparative Example 2-2

A PVDF pipe, having an outside diameter of 20 mm, an inside diameter of19 mm, a thickness of 0.5 mm and a length of 600 mm, was produced as aclad region by extrusion molding of poly-fluorine-vinylidene (PVDF),“KF-#850” manufactured by Kureha Chemical Industry, Co., Ltd,.

A polymerizable composition containing a monomer mixture of deuteratedmethyl methacrylate, MMA-d8, and isobornyl methacrylate in which amethacrylate moiety was deuterated, IBXMA-d5, both of them being removedhydroquinone monomethyl ether as an polymerization inhibitor and reducedwater content by 80 ppm, and the MMA-d8 to IBXMA-d5 weight ration was7/3; 0.5 wt %, with respect to the monomer mixture weight, of BPO as apolymerization initiator and 0.28 wt %, with respect to the monomermixture weight, of n-laurylmercaptan as a chain transfer agent; wasdirectly poured into the hollow region of the obtained PVDF pipe whilebeing filtered through a membrane filter, based on tetrafluoroethylene,having a pore size of 0.2 μm. The PVDF pipe poured the polymerizablecomposition into was placed in the water bath at 80 degrees Celsius andthe polymerizable composition was shaken and pre-polymerized at 80degrees Celsius for 2 hours. Subsequently, the pipe was heldhorizontally and rotated at a speed of rotation of 3,000 rpm, to carryout polymerization of the polymerizable composition at 80 degreesCelsius for 3 hours. After that, heat treatment was carried out at 100degrees Celsius for 24 hours. Thus, an outer core region formed of thecopolymer was produced.

Next, a monomer mixture of MMA-d8 and IBXMA-d5, both of them beingremoved hydroquinone monomethyl ether as a polymerization inhibitor andreduced water content by not greater than 80 ppm, and the MMA-d8 toIBXMA-d5 weight ratio was 7/3; and 12.5 wt %, with respect to themonomer mixture weight, of a compound 2-(4) or 2-(6) or comparativecompound 2-(19) described above as a dopant were mixed, thereby toobtain a mixed solution. The mixed solution was directly poured into thehollow region of the obtained hollow cylinder while being filteredthrough a membrane filter, based on tetrafluoroethylene, having a poresize of 0.2 μm. 0.016 wt %, with respect to the monomer mixture weight,of PBD as a polymerization initiator and 0.27 wt %, with respect to themonomer mixture weight, of n-laurylmercaptan as a chain transfer agentwere added to the mixed solution. The chain transfer constant ofn-laurylmercaptan in this system was 0.8. A cylinder poured the mixedsolution into was housed in a glass tube having a diameter larger by 9%than the outer diameter of the cylinder, and was then allowed to standvertically in a pressure polymerization reactor. The inner atmosphere ofthe pressure polymerization reactor was then purged with nitrogen,pressurized up to 0.2 MPa, and the heat polymerization was allowed toproceed at 100 degrees Celsius for 48 hours and subsequently 120 degreesCelsius for 24 hours with keeping the pressured atmosphere to therebyobtain the preform.

The obtained preform observed when the polymerization completed wasfound to have no air bubbles contained therein which possibly introducedby volume shrinkage. The preform was drawn by thermal drawing at 230degrees Celsius so as to form a plastic optical fiber having a diameterof approx. 500 μm. The preform was not found to include air bubblesduring the drawing, which contributed to successfully obtain the fiberof 300 m long in a stable manner.

The transmission loss for 850 nm light source of each obtained opticalfibers was measured. And after being left under condition of that thetemperature was 25 degrees Celsius and RH was 95%, the transmission lossfor 850 nm light source of each obtained optical fibers was measured.The increase of transmission loss was shown in Table 2-2.

Next, the outer surface of each obtained optical fibers in the Examplesand the Comparative Examples was coated with polyethylene, therebyforming a primary coating layer having a thickness of 0.75 mm, andsecondary coated with polyethylene containing 3% of carbon, therebyforming a secondary coating layer having a thickness of 0.75 mm. Bendingtests were performed respectively for those. The bending tests wereperformed according to the method disclosed in JPA No. 1995-244220.Specifically, the coated fiber was given bending once by being wrappedby 90 around a mandrel having a 60 mm diameter, and the lighttransmission loss due to bending was measured. The maximum values oftransmission loss due to bending were shown in Table 2-2. TABLE 2-1Increase of Transmission Transmission transmission loss due to Loss lossbending Dopant [dB/km] [dB/km] [dB] Example (4) 510 290 0.7 2-1 Example(6) 495 280 0.7 2-2 Comparative (19)  770 350 1.0 Example 2-1

TABLE 2-2 Increase of Transmission Transmission transmission loss due toLoss loss bending Dopant [dB/km] [dB/km] [dB] Example (4) 420 220 0.052-3 Example (6) 405 210 0.05 2-4 Comparative (19)  680 270 0.07 Example2-2

As shown in the Table 2-1 and 2-2, it was found that when the dopanthaving the SP value greater than 10.9, i.e. the comparative compound(19), was used, both of the primary transmission loss and the increaseof transmission loss after being left under the humidity and the heat;and when the compound 2-(4) or 2-(6) having the SP value not greaterthan 10.9, was used, the good results were obtained. It was also foundthat when the fiber was coated, the good result, such as lowtransmission loss caused by bending, was obtained as the above.

INDUSTRIAL APPLICABILITY

In one aspect, the present invention can provide a polymerizablecomposition capable of producing optical members for 850 nm wavelengthand an optical member having low transmission loss at 850 nm with lowcost.

In another aspect, the present invention can provide a polymerizablecomposition capable of producing optical members having low transmissionloss at 850 nm and a good moisture-heat-resistant property and anoptical member having a low transmission loss at 850 nm and a goodmoisture-heat-resistant property.

1. A polymerizable composition for producing an optical member for 850nm wavelength comprising: a polymerizable monomer composition, apolymerization initiator, and a compound, having a different refractiveindex from that of the polymerizable monomer composition, whosestructure has a benzene ring substituted by a substituent having aHammett value of not greater than 0.04 or by plural substituents havingan average value of Hammett values thereof of not greater than 0.04. 2.The polymerizable composition of claim 1, wherein the polymerizablemonomer composition comprises at least one selected from the groupconsisting of esters of a propenoic acid and esters of derivativesthereof in a major proportion.
 3. The polymerizable composition of claim2, wherein the polymerizable monomer composition comprises at least oneselected from the group consisting of esters of a (meth) acrylic acidand esters of derivatives thereof in a major proportion.
 4. Thepolymerizable composition of claim 1, wherein the polymerizable monomercomposition comprises at least one selected from the group consisting ofcompounds including a C—F bond.
 5. The polymerizable composition ofclaim 1, wherein the polymerizable monomer composition comprises atleast one selected from the group consisting of compounds including aC-D (deuterium) bond.
 6. An optical member produced by polymerization ofa composition of claim 1, so as to form a region having a gradedrefractive index.
 7. An optical member for 850 nm wavelength comprises:a polymer composition comprising at least one polymer selected from thegroup consisting of (meth)acrylates base polymers and a compound havinga different refractive index from that of the polymer compositionwherein the compound has an absorption peak attributed to a fourthovertone of C—H bond stretching vibration in a benzene ring at notshorter than 875 nm.
 8. The optical member of claim 7 wherein thecompound is selected from the group consisting of: Formula (1)

wherein R¹ to R¹⁰ respectively represent a hydrogen, an alkyl, analkenyl, an alkyloxy, an alkenyloxy, or dialkylamino provided that atleast four of them represent an alkyl, alkenyl, alkyloxy, alkenyloxy ordialkylamino.
 9. The optical member of claim 7, which comprises a regionhaving a graded refractive index.
 10. The optical member of claim 9,which comprises a region having a graded refractive index along thedirection from the center to the outside.
 11. A polymerizablecomposition for producing an optical member comprising: a polymerizablemonomer composition comprising at least one selected from the groupconsisting of: Formula (2)

wherein X¹ is hydrogen (H) or deuterium (D) wherein two X¹ may be sameor different; Y¹ represents H, D, CH₃ or CD₃; and R¹ represents a C₇₋₂₀alicyclic hydrocarbon group; a polymerization initiator, and a compound,having a different refractive index from that of the polymerizablemonomer composition and having a solubility parameter of not greaterthan 10.9, whose structure has a benzene ring substituted by asubstituent having a Hammett value of not greater than 0.04 or by groupshaving an average value of Hammett values thereof of not greater than0.04.
 12. The polymerizable composition of claim 11 wherein thepolymerizable monomer composition comprises an alicyclic hydrocarbonmethyl methacrylate and methyl methacrylate in a major proportion. 13.The polymerizable composition of claim 12 wherein the polymerizablemonomer composition comprises at least one compound including a C-Dbond.
 14. An optical member produced by polymerization of a compositionof claim 11, so as to form a region having a graded refractive index.15. The optical member of claim 14 comprising a core region having agraded refractive index, which is produced by polymerization of acomposition of claim 11 and a clad region cladding the core region. 16.The optical member of claim 15, wherein the core region having a gradedrefractive index along the direction from the center to the outside. 17.The optical member of claim 15, wherein the clad region is essentiallyformed of a polymerizable monomer composition comprising a sameingredient or same ingredients in a major portion as those of the coreregion.
 18. The optical member of claim 15 which is an optical fiber, alight guide or an optical lens.
 19. A process for producing an opticalmember comprising a step of polymerizing the polymerizable compositionof claim
 1. 20. The process of claim 19, wherein, in said step ofpolymerizing, the polymerization temperature is 50 degrees Celsius orabove.